1. Field of the Invention
This invention relates to layered materials, especially those which have useful catalytic and adsorbent properties. More particularly, it relates to a process for increasing the interlayer distance of such materials by incorporating pillars comprising inorganic substances. This process introduces pores or empty spaces between the layers thereby enhancing the sorptive capacity and catalytic properties of the material.
2. Description of the Related Art
In the preceding two decades, a new class of two-dimensional porous materials has been synthesized from smectite clay minerals. Smectite clays are able to swell in water because of their low layer charge, and thus easily intercalate organic guest molecules or large inorganic polymers. In general, the organically pillared structures suffer from the thermal instability of the organic component. The temperature sensitivity of these materials limits their utility as catalysts. Since a major incentive for preparing these materials is to produce new, catalytically active, large pore structures, robust, temperature stable pillars are required. Consequently, attention has focused on the use of the aluminum Keggin ion [Al.sub.13 O.sub.4 (OH).sub.24 .multidot.12H.sub.2 O].sup.7+ and the zirconium tetramer [Zr(OH).sub.2 .multidot.12H.sub.2 O].sub.4.sup.8+ as pillars. A large literature has developed, details of which are given in recent reviews (see, e.g., T. J. Pinnavaia, Science, 220, 365 (1983) and A. Clearfield in "Surface Organo-metallic Chemistry," Proc. of NATO Workshop, May 25-30, 1986, Le Rouret, Fr. 1).
Clays as they occur in nature are rocks that may be consolidated or unconsolidated. Clays are composed of extremely fine crystals or particles of clay minerals with or without other rock or mineral particles. These crystals or particles are often colloidal in size and usually platy in shape. The clay minerals, mostly phyllosilicates, are hydrous silicates of aluminum, magnesium, iron, and other less abundant elements.
The very fine particles yield very large specific-surface areas that are physically sorptive and chemically surface-reactive. Many clay mineral crystals carry an excess negative electric charge owning to internal substitution by lower valent cations, and thereby increase internal reactivity in chemical combination and ion exchange. Catalysts made from various clay minerals are extensively used, e.g., in the cracking of heavy petroleum fractions. These catalysts are produced from halloysites, kaolinites, and bentonites composed of montmorillonite.
Smectites (montmorillonites) are the 2:1 clay minerals that carry a lattice charge and characteristically expand when solvated with water and alcohols, notably ethylene glycol and glycerol. In earlier literature, the term montmorillonite was used for both the group (now smectite) and the particular member of the group in which magnesium is a significant substituent for aluminum in the octahedral layer.
The cation-exchange capacity of smectite minerals is notably high, 80-90 meq or higher per 100 grams of air-dried clay. The crystal lattice is obviously weakly bonded. Moreover, the lattice of smectites is expandable between the silicate layers so that when the clay is soaked in water it may swell to several times its dry volume (e.g., bentonite clays).
The principal clay minerals are kaolinite, montmorillonite, and illite. These are actually three families of minerals since kaolinite has several modifications and since isomorphous substitution occurs in the latter two giving rise to other compositions having different mineral names. Closely associated with the above minerals are gibbsite, Al(OH).sub.3, diaspore, HAlO.sub.2, and bauxite (of indefinite composition but usually given as Al.sub.2 O.sub.3 .multidot.2H.sub.2 O which is an intermediate between the first two). All clays have as the major constituents one or more of the above minerals or minerals of the above families.
Montmorillonites have the general formula: X.sub.y Al.sub.2 (Al.sub.y Si.sub.4-y O.sub.10)(OH).sub.2 where X is usually Na, Mg or Al. The montmorillonite group includes the minerals montmorillonite, nontronite, beidellite, hectorite, and saponite. The latter two are trioctahedral and the other three are dioctahedral. Extensive substitutions occur in the octahedral sites as well as substitutions of aluminum for silicon in the tetrahedral sites. Montmorillonite absorbs water readily with accompanying swelling. It is the principal mineral in bentonite and accounts for its high plasticity and usually very sticky nature. The structure of this group of minerals is like that of talc and is classified with the sheet or layer silicates.
Stable pillared interlayered clay compositions have been prepared by reacting smectic type clays with polymeric cationic hydroxy metal complexes of metals such as aluminum, zirconium and/or titanium. Vaughan et al., U.S. Pat. No. 4,176,090, describes a process in which naturally occurring or synthetic smectite type clays are reacted with aluminum chlorohydroxide complexes ("chlorhydrol"), and then heated to convert the hydrolyzed polymer complex into an inorganic oxide.
The general procedure described in Vaughan et al. comprises mixing a smectite clay with an aqueous solution of the polymeric cationic hydroxy metal complex. The mixture of clay and metal complex is maintained at a temperature up to 200.degree. C. for up to 4 hours. The reacted clay solids are then recovered and heated at a temperature of 200-700.degree. C. to decompose the hydrolyzed metal complex to a pillar of inorganic oxide. The introduction of discrete/non-continuous inorganic oxide particles between the clay layers is said to produce pillared interlayered clays possessing a unique internal micropore structure.
Another patent to Vaughan et al., U.S. Pat. No. 4,248,739, describes a similar preparation of pillared interlayered clays which uses a polymeric cationic hydroxy inorganic metal complex having a molecular weight in excess of 2000. The complex may be formed by a hydrolysis-polymerization reaction of chlorhydrol. This reaction may be base-catalyzed. A smectite clay is mixed with an aqueous solution of the high molecular weight polymeric cationic hydroxy metal complex, polymer or copolymer such that the weight ratio of clay to metal complex is from 3 to 1.
In the method of Vaughan et al., an expandable layer-type clay (smectite) is reacted with a polymeric cationic hydroxy metal complex of aluminum and/or zirconium. Upon calcination, the interlayered metal complex is decomposed to form "inorganic oxide pillars" between the expanded clay layers which are separated by a distance of about 6 to 16 angstroms. The resulting pillared interlayered clay products are said to possess a unique interconnected internal micropore structure in which more than half of the pores are less than about 30 angstroms in diameter.
Apparently, while the interlayered clay products possess some degree of ion exchange capacity, the ion exchange capability of the calcined interlayered clay product is not equivalent to the ion exchange capacity of the parent clay. In U.S. Pat. No. 4,271,043, Vaughan et al. describe a method for increasing the ion exchange capacity of such pillared interlayered clays which comprises treating the calcined product with a base, such as aqueous solutions of alkali metal or ammonium hydroxides or carbonates, or a gaseous basic reactant such as ammonia.
U.S. Pat. No. 4,238,364 to Shabtai describes cracking catalysts consisting of highly acidic forms of cross-linked smectites. The preferred method of preparation for these catalysts includes preparing acidic forms of the smectite (usually montmorillonite) and thereafter performing a non-stoichiometric (i.e., partial) cross-linking of the acidic smectite with oligomeric species of aluminum hydroxide. The cross-linked material is subsequently stabilized by heat treatment.
The process described in U.S. Pat. No. 4,238,364 "is performed with preservation of the structure of the smectite unit layers, as the cross-linking step affects only the interlamellar space of the smectite. This is fundamentally different from the methods used in the preparation of clay-based and clay/gel-based zeolite cracking catalysts, since in these cases the clay (smectite) structure is subjected to drastic structural changes during the catalyst preparation process."[col. 4, lines 25-32]
Shabtai describes the preparation and properties of non-functionalized cross-linked frameworks in U.S. Pat. No. 4,216,188. The process of preparing the cross-linked montmorillonite molecular sieves comprises interaction between montmorillonite, in the form of a colloidal solution containing fully separated unit layers, and a cross-linking agent, consisting of a buffered and aged colloidal solution of a metal hydroxide, dispersed in the form of low molecular weight oligomers.